(1) Field of the Invention
The present invention relates to a developing device which performs development of the static latent image formed on the static latent image bearer, by rotating a developer support that incorporates a magnetic field forming means having multiple magnetic poles fixed at predetermined positions and that supports a developer thereon so as to supply the static latent image with the developer.
(2) Description of the Prior Art
When a dual-component developer is used, toner concentration unevenness will occur in the dual-component developer layer after development of a static latent image in the developing area. To deal with this, in a conventional developing device for dual-component developer, two magnetic poles, namely the first and second poles, of a similar polarity are arranged adjacent to each other inside the developer sleeve using fixed magnets, on the downstream side of the developing portion with respect to the rotational direction of the developer sleeve, so that a repulsive magnetic field formed by these two magnetic poles will remove the developer from the developer sleeve. The thus removed developer is agitated by a rotational agitator disposed in the repulsive magnetic field to make the toner concentration uniform, and then the developer with the toner density made uniform is conveyed by the rotating agitator to the developer sleeve on its second magnetic pole side. Thus, the density unevenness in the developed image due to development hysteresis can be prevented.
However, under the formation of a repulsive magnetic field alone, it is not possible to completely remove the developer from the developer sleeve, so some part of the developer is conveyed whilst adhering to the developer sleeve, from the first magnetic pole side to the second magnetic side. Particularly, when the toner is a micro particle toner and/or the magnetic carrier is a micro particle carrier, the tribo-charge amount on the developer per unit weight becomes relatively large so that the developer adheres to the developer sleeve with a greater electrostatic attraction. Therefore, it is difficult for the repulsive magnetic field to fully remove the developer from the developer sleeve.
So that, when the developer having passed through the developing area cannot be removed from the developer sleeve and hence re-enter the developing area, the developed image may have density unevenness and cannot offer a high, fine quality. Further there is the problem in that if the developing device has been run for a long period of time without the developer fully removed from the developer sleeve, degradation of the developer is accelerated causing density lowering and background fogging in the developed image.
As countermeasures against these problems, for example, Japanese Patent Application Laid-Open Hei 6 No.194962 has offered a method in which the developer is removed from the developer support by using a magnetic brush formed between the developer support and the magnetic element, by providing a magnetic element in the developer flow path under the influence of a repulsive magnetic field. Another method has been also proposed in Japanese Patent Application Laid-Open Hei 5 No.289523, in which the developer is removed from the developer support by providing a cleaning member in the developer flow path in the repulsive magnetic field so that the cleaning member comes into frictional contact with the developer support.
However, a small-diametric developer sleeve configuration in combination with the scheme of Japanese Patent Application Laid-Open Hei 6 No.194962 where the removal of the developer is performed by using lowering of the developer support's retention force by formation of a repulsive magnetic field, has a disadvantage in terms of cost because the fixed magnets incorporated in the developer sleeve need to be magnetized with a high precision.
Further, provision of a magnetic element or a cleaning member in the developer flow path will disturb the flowing state of the developer in the developing hopper, inducing a high risk of degradation of the image quality. In particular, in the illustration of Japanese Patent Application Laid-Open Hei 6 No.194962, the magnetic element is located between the developer sleeve and rotational agitator. However, originally the developer conveyance pressure and the toner concentration are high at this point, so that the conveyance pressure is further increased if a magnetic element is laid out in such a position, which may possibly cause problems of toner aggregation, toner filming over the developer sleeve surface and/or spent toner.
Moreover, the arrangement of a cleaning member in direct and frictional contact with the developer support surface, produces excessive stress on the developer, which may result in increasing of the driving torque and heat generation of the developing unit.